A New View of the Highly-Ionized Gas in the Milky Way

Type Conference Paper
Names Nicolas Lehner, W. Zech, J. Howk, B. D. Savage
Proceedings Title Bulletin of the American Astronomical Society
Conference Name American Astronomical Society, AAS Meeting #215, #415.12
Volume 42
Pages 262
Date January 1, 2010
URL http://adsabs.harvard.edu/abs/2010AAS...21541512L
Library Catalog NASA ADS
Abstract Using the highest resolution mode of STIS (R>114,000, 1.3-2.7 km/s), we provide new insight into the physical properties of the highly ionized plasma probed by the "high ions" (CIV, SiIV, NV). We use a high-quality sample of 38 sightlines probing both the Galactic disk and halo gas. Contrary to common belief, we find that 40-50% of the individual components of SiIV and CIV are narrow, implying cool temperatures of T<50,000 K. Very few of these narrow components are consistent with photoionization by OB-type stars; photoionization from radiatively cooling hot plasma or the remains of a hot radiatively cooling gas is a more likely scenario. The remaining 50% is consistent with >100,000 K plasma, but the contribution from non-thermal motions can be significant. Thus, the broad components may be hot, collisionally ionized gas, but some fraction may also be cooler gas with significant non-thermal components. We finally compare the properties of the highly-ionized gas with other gas-phase tracers (e.g., AlIII, SiII, X-rays) in order to decipher the relationship between the various phases of the interstellar gas.
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